An optical parametric oscillator (OPO) is a light source emitting radiation with properties comparable to that of a laser. OPOs are nonlinear devices that split short wavelength pump photons into two longer wavelength photons, namely signal and idler photons. The wavelengths of the signal and idler photons are not independent from each other, but may be tuned in wavelength.
As shown by FIG. 1, an OPO converts an input laser wave (the “pump”) with frequency ωp into two output waves of lower frequency (ωs, ωi) via second-order nonlinear optical interaction. The sum of the frequencies of the output waves is equal to the input wave frequency: ωs+ωi=ωp. For historic reasons, the output wave with the higher frequency ωs is called the signal, and the output wave with the lower frequency ωi is called the idler. Because the OPO does not convert all the input energy into the signal and idler, a residual pump wave is also output.
OPOs need an optical resonator, but in contrast to lasers, OPOs are based on direct frequency conversion in a nonlinear crystal rather than from stimulated emission. OPOs exhibit a power threshold for an input light source (pump), below which there is negligible output power in the signal and idler bands.
OPOs include an optical resonator (cavity) and a nonlinear optical crystal. The optical cavity is an arrangement of mirrors that forms a resonator for light waves. Light confined in the cavity is reflected multiple times resulting in a multi-pass through the nonlinear crystal. The optical cavity serves to resonate at least one of the signal and idler waves. In the nonlinear optical crystal, the pump, signal and idler beams overlap.
While conventional lasers produce limited fixed wavelengths, OPOs may be desirable because the signal and idler wavelengths, which are determined by the conservation of energy and momentum (via phase matching), can be varied in wide ranges. Thus it is possible to access wavelengths, for example in the mid-infrared, far-infrared or terahertz spectral region, which may be difficult to obtain from a laser. In addition, OPOs allow for wide wavelength tunability, for example, by changing the phase-matching condition. This makes OPOs a useful tool, for example, for laser spectroscopy.
A limitation is that OPOs generally require a pump source with high optical intensity and relatively high spatial coherence. An OPO is usually pumped by a laser, but the direct use of a laser diode is usually not easily possible. Therefore, the system becomes relatively complex, for example consisting of a diode-pumped solid-state laser or laser devices utilizing fiber-amplification, and the actual OPO. However, such pumps producing high quality beams may be expensive, large, and produce considerable heat. For example, OPOs are typically pumped using coherent narrow linewidth lasers, for example, at 1064 nm. Therefore, there is a need in the industry to address one or more of the above mentioned shortcomings.